1. Field of the Invention
The present invention relates to the field of semiconductor processing and more specifically to a method and apparatus for monitoring or the end point detecting a process.
2. Discussion of Related Art
As dynamic random access memories (DRAMs) devices increase in storage density from 16 Mb to 64 Mb and beyond there is a need to maintain charge storage capabilities with a decrease in memory cell size. This requirement has been met by increasing the surface area of the storage capacitor within the memory cell. Several complex storage electrode designs have been implemented, including thin and cylindrical xe2x80x9ccrownxe2x80x9d capacitor structures in order to obtain increased storage surface area.
Another approach, as illustrated in FIG. 1a, is to form a capacitor 100 having a lower capacitor electrode 102 formed from a polysilicon film having a roughened surface 104. A polysilicon film having a roughened surface can be generated by forming a polysilicon film with hemispherical grains (HSG silicon). Because electrode 102 has a rough surface, the surface area between top electrode 106 and bottom electrode 102 is increased enabling more charge storage for a given cell size. A polysilicon storage electrode 102 with a rough surface can increase the capacitor area by more than 2xc3x97.
The most wide spread approach for forming a polysilicon film with a rough surface or hemispherical grains has been to deposit a thin (less than 1,000 angstroms) rough polysilicon film in the batch furnace, such as shown in FIG. 1b. The batch furnace 120 illustrated in FIG. 1b is a low pressure chemical vapor deposition (LPCVD) system having a chamber 110 which includes a boat 111 carrying a batch (approximately 100) of substrates. A gas feed from a gas source 113 is controlled by a controller 114 and enters the chamber 110 from a gas inlet port 115. The gas feed is maintained across the substrates 112 in the direction of the arrows. The low pressure in the chamber 110 is maintained by exhaust system 116. Because the concentration of fed gases can decrease the flow toward the exhaust system 116 the chamber also includes three separately controlled heater eliminates 117 that provide temperature variations in chamber 110 to compensate for variations of concentration of the reactant gases within chamber 110.
A problem associated with present techniques of forming HSG silicon is that there is presently no way to monitor or end point detect the formation of HSG silicon. It is to be appreciated that there is a relatively small process window (time) in which HSG grains have optimum size and shape in order to provide a maximum increase in the capacitor surface area. If the process time is too short too little HSG will form resulting in a insufficient increase in electrode surface area. If HSG process time is too long, the gaps between adjacent grains begin to fill in resulting in a smoothing of the roughened surface and a decrease in electrode surface area. Thus, without a technique to monitor and/or end point detect the formation of HSG silicon, HSG silicon processes will cause poor substrate to substrate uniformity and potentially be unmanufactureable.
Thus, what is desired is a method and apparatus for monitoring and/or end point detecting the formation of HSG silicon.
A method and apparatus for monitoring or end point detecting a process is described. According to the present invention a surface characteristic of the substrate is continually measured while processing the substrate. A change in the surface characteristic is utilized to monitor the process or to signal an end to the processing of the substrate.
In an embodiment of the present invention, a change in a surface characteristic of a substrate is utilized to monitor or to signal the end to a process for forming hemispherical grain (HSG) silicon. In such a process a substrate having an external amorphous silicon film is placed in a process chamber. The substrate is then heated to form hemispherical grains from the amorphous silicon film. While heating the substrate a surface characteristic is continually monitored. A predetermined change in the surface characteristic is utilized to signal the completion of hemispherical grain silicon formation and therefore an end to the heating step.
In an embodiment of the present invention the emissivity of a substrate is utilized to monitor or to signal the end point of a process used to form HSG silicon. According to this embodiment of the present invention a substrate having an amorphous silicon film is placed in a process chamber and heated to form hemispherical grain (HSG) silicon from the amorphous silicon film. While heating the substrate the emissivity of the substrate is continually monitored. A predetermined change in the emissivity of the substrate is utilized to monitor the process or to signal an end to the heating step.
In another embodiment of the present invention the temperature of a substrate is utilized to monitor or to signal the end point of a process used to form HSG silicon. According to this embodiment of the present invention a substrate having an amorphous silicon is placed in a process chamber and heated to form hemispherical grain silicon from the amorphous silicon film. While heating the substrate with a constant amount of heat the temperature of the substrate is continually monitored. A predetermined change in the temperature of the substrate is utilized to monitor the process and/or to signal an end to the heating step.
In still another embodiment of the present invention while processing a substrate, the temperature of the substrate is monitored utilizing a temperature measurement device which is substantially independent of the substrates emissivity. Additionally while processing the semiconductor substrate the temperature of the substrate is also monitored utilizing a temperature measurement device which is dependent on the substrate""s emissivity. While processing the substrate, the difference between the temperature measured by substantially emissivity independent measurement device and the temperature measured by emissivity dependent measurement device is calculated. A change in the difference between the temperature measured by the substantially emissivity independent measurement device and the temperature measured by the emissivity dependent measurement device is then utilized to monitor the process and/or to signal an end to the processing step.
In still yet another embodiment of the present invention a substrate is heated with a closed loop temperature control system which through temperature feedback maintains the substrate at a constant temperature. The amount of power utilized by the temperature control system is continually monitored while heating the substrate. A change in the amount of power required by the temperature control system is used to monitor the process and/or to signal an end to the heating step
Other embodiments and features of the present invention will become obvious from the detailed description which follows.